Method of spheroidizing hypereutectoid steels



METHOD OF SPHEROIDIZING HYPEREUTECTOID STEELS Adolph J. Lena, Sarver, and Glenn W. Bush, State College, Pa., assignors to Allegheny Ludlnrn Steel Corporation, Brackenridge, Pa., a corporation of Pennsyivania No Drawing. Application January 3, 1957 Serial No. 632,957

4 Claims. (Cl. 14821.5)

This invention relates to an improvement in the method of producing steel and in particular to an improvement in the method of producing a spheroidized hypereutectoid steel.

Heretofore the production of spheroidized carbon steels has been a time consuming and costly process. in practicing the known processes, coils of high carbon steel have usually been spheroidized either by a long time annealing treatment or by a cyclical annealing treatment. Such known processes require heat treatment times ranging from about 8 hours to about 50 hours or more to spheroidize such steels. Such methods of producing spheroidized carbon steel are by necessity batch type processes because the length of time required for the desired metallurgical transformations is too long to be adapted to a continuous process. in practice the known batch type processes are usually applied to coils of steel with accompanying inherent disadvantages.

Thus upon heating the coils, the outside layers of the coils being processed by either of the known methods reach the spheroidizing temperature long before the center of the coil. Also upon cooling, the outside layers of the coil cool more quickly than the center of the coil. Hence the unequal heating and cooling rates result in different sections of the coil being at different temperatures for various periods of time. Since the spheroidization of steel depends upon the temperature and the time at temperature, the microstructures of the coils produced by either of the known methods vary considerably within the coil and are diificult to control or reproduce. Upon subsequent application of this steel to a commercial use, for example a band saw blade, the steels are found to be non-uniform in their response to heat treatment with the result that the properties of the steel v-aryafrom one end of the coil to the other.

An object of this invention is to provide a method for continuously processing hypereutectoid steels to produce a substantially uniform mierostructure therein.

Another object of this invention is to provide a method for processing hypereutectoid steels to produce a substantially uniform spheroidized carbide particle varying in size between pinpoint and 2.2 mm. in diameter when such microstructure is resolved at 1500 diameters and substantially uniformly distributed within a ferrite matrix.

A more specific object of this invention is to provide a method for continuously or semi-continuously producing hypereutectoid steels having substantially uniform properties, said process having as essential steps an austenitizing heat treatment, an isothermal transformation heat treatment and a spheroidizing heat treatment.

Other objects of this invention will become apparent when read in conjunction with the following description.

In general, hypereutectoid steels may be spheroidized after an austenitizing heat treatment from any one of three basic microstructures which are formed when the steel is cooled from the austenitizing heat treatment. Thus a steel may be austenitized and cooled at a very slow rate so that the transformation product consists of a mi- 2,824,826 Patented Feb. 25, 1958 crostructure consisting of proeutectoid cementite and a coarse to fine lamaller pearlite. This structure can be spheroidized by heating the steel to a temperature slightly below the critical temperature and holding the steel at this temperature for long periods of time ranging between 8 and 50 hours ormore. This treatment usually results in a steel having irregularly shaped carbide particles, extremely large in size and nonuniformly distributed throughout the ferrite matrix. On the other hand, steel may be quenched from the austenitizing heat treatment at a rate sutficiently fast to transform the structure of the steel to martensite, an extremely hard and brittle phase of steel. If the quenching is sufficiently fast there is little, if any, proeutectoid cementite visible within the microstructure. With the steel in the martensitic condition, it is so hard and brittle that it is impossible to process the steel in coil or strip form in any of the known continuous or semi-continuous manners without encountering cracking or breaking. It is therefore apparent that spheroidization of such steel during such continuous or semi-continuous processing is not commercially feasible. Thus in either of the foregoing described processes the steel is not adaptable to processing on a continuous or semi-continuous basis.

On the other hand, we have found that through the use of the method of this invention, to be more fully described hereinafter, We have been able to produce a substantially uniform distribution of spheroidizecl carbide particles having a size varying between pinpoint and 2.2 mm. in diameter as examined and measured at a magnification of 1500 diameters. It has been found that the classification of the carbide particles as such is most conveniently accomplished by polishing and etching a sample of the steel and viewing the microstructure' thereof at magnification of 1500 diameters. Classification under such circumstances appears to be most advantageous from the standpoint of producing a steel which has substantially uniform properties and is very well suited to commercial applications, for example, in the production of band saw blades.

The method of this invention is applicable to steels which are termed hypereutectoid, a term common in the metallurgical art, said term referring to a steel having a carbon content in excess of the eutectoid composition. It will, of course, be apparent that the carbon content for a hypereutectoid steel will vary, for example, in plain carbon steels from about 0.80% to about 1.3% carbon. As is well known, the eutectoid composition may readily occur at carbon contents below about 0.80% where other alloying elements are present. In general, however, while the process of this invention could be adapted to most hypereutectoid steels, it is particularly effective for producing uniform results in a plain carbon hypereutectoid steel having a carbon content within the range between about 1.2% and 1.3%.

These steels may be made in any of the well-known steel mill manners, the steps of which are well known in the art and will not be described in detail. It is sufficient to say that a melt of such steel is cast into ingots which are thereafter hot formed in any desired manner usually, however, byhot rolling into the form of strip. The strip which usually forms the starting material for the method of this invention may be in the hot rolled or the hot rolled and annealed condition. However, it has been found quite advantageous to continuously normalize the steel strip regardless of the condition of the steel in order to provide a more uniform starting material. By continuously normalizing the hot strip material, a substantially uniform response to the process of this invention is obtained from heat to heat. The normalizing may convenientlytbe accomplished by heating the steel strip to a temperature in the range betw "for a time period of about '10 minutes. malizing, a Substantially uniform starting material is 'tained which is substantially unaffected by any of same heat of steel;

1500 F." and 1600-F. for a period of time ranging between 150 minutes and 250 minutes per inch of strip thickness of the hot rolled'strip material. Preferably,

processing variables which may have occurred in normal production of hot rolled strip from ingots in The steel 'in the hot rolled or the hot rolled and anhealed conditionand preferably iri'the normalized conditicna's described" hereinbefore is subjected to an austenitizing heat-treatment by heating to a temperature within the rangeb'etween 1900" and 200051 The austenitizing heat treatment .is most conveniently accomplished' by continuouslyaustenitizing the strip material having a thickness of up to l25 inch but usually in the range between 0.065 and 0.040 inch at a temperature in the range given for a time period rangingbetween 60 minutes and 125 minutes perinch of strip'thickness.

The auste'nitizing heat'treatment is very critical in the process of this invention and as such-three criteria must be met in order for the process of this invention to be .used on a continuous or a semi-continuous, basis. These criteria are: (1) Theheat treatment times must not be excessive for'the process to be continuous; (2) all of the'carbide particles in the steel regardless of the conditicn of the starting material, as described hereinbefore,

'mustbe completely dissolved; at the temperature 'em-;

within the time at temperaturein the aus- 35 ployed and tenitizing heat treatment to thereby form a substantially homogeneous austenite, and (3) excessive grain growth must be prevented. It has been found that these con ditic-ns can only be met when the steel is austenitized at a temperature in the range between 1900 F. and.

2000 F. in the time'period specified. Austenitizing temperature below about 1900 F. within a time period for the process to be adapted to a continuous or semicontinuous operation is not effective for dissolvin'g all of the carbide particles, hence a heterogeneous austenite is formed. f Temperatures much in excess of 2000 F.

result in an excessive grain growth with the result that in addition to the excessively long periods of 'tir'ne neces-' sary in order to spheroidize thesteel' which results in the formationiof an irregular carbide particle substantially non-uniformly distributed Within the ferrite matrix, the excessive 'grain'size materially affects the physical'p'rop ertie's to such an extent that the steel is unsuited for use in its intended application, for example, band saw blades.

In particular, it has been found that if a hypereutectoid steel having'a thickness of up to'about 0.125 inch but preferably in the range between 0.040 inch and 0.065

inch is austenitiz'ed at a temperature withinthe range between 1925? F. and 1950 F. for a' time period of about 5 minutes substantially uniform results are obtained from heat to heat having substantially the same chemical analysis. Added advantages are also noted in that austenitizing' the'se steels'at the temperature in the range between 1900 F1 and 2000'F. for a time period ranging between about 60 minutes and about 125 minutes per inch of thickness is effective for preventing excessive deca'rburization of the steel during the austenitizing heat treatment.

Following the austenitizing heat treatment, the by pereutcctoid steel is isothermally transformed by quench f 'ing the steel from theaustenitizing heat treatment temperature to a temperatureiri the range between about 7 ing heat treatment temperature. 'These structures con-- depending upon the rateof cooling from the a'ustenitizsist predominantly of coarse to fine pearlite, coarse to fine bainite or a substantially completely martensitic structure, respectively, as the temperature to which the steel is quenched is decreased. It is desirable in the process of this inventionv to isothermally transform the austenite to bainite. This is quite advantageous in. the process of this invention becauseth'e" timei period'reeuired to transform the steel to the bainite structure as. well as the time required to transformthe bainite" to a structure of spheroidized carbide in a ferrite matrixis substantially less than the a time required to transform a pearlitic structure to a structure containingspheroidized carbide in a ferrite 'matrix. In: addition, substantially uniform results are obtained in the steel, which results are readily reproducible. The steel-having a bainite structure possesses a hardness within the range between about 40 R and 53 R high strength and toughness butis-not sufficiently ductile to allow it to be substantially cold roll reduced at this stage of the heat treatment However, the steel has sufiicient ductility to allow it tobe adapted to a continuous or semi-continuous operation. t On the other hand, the steel, if transformed: to martensite, is extremely hard and brittle and as such it possesses little, if any, ductility and in most cases the steel is sobrittle as to make it substantially impossible of adaptatron to a continuous or even semi-continuous process.

These difiiculties are avoided when the steel is isothermally transformed to bainite. In addition, added advantages are noted in that the steel may be coldroll reduced intermediate a cyclical spheroidization heat treatment, to be more fully described hereinaften;

It is preferred to effect the isothermal transformation by quenching the steel from the austenitizing heat treatment temperature in an isothermal transformation bath having a temperature in the range between 725 F. and 750 F. Such isothermal transformation bath usually consists of a bath of molten salt or lead. Substantially uniform results are obtained when the steel is transformed in this temperature range and thereafter spheroidized as will be more fully described. In particular, a steel, haying a thickness of up to about 0.l25 inch but preferably within the range between 0:040 inch and 0.065 inch' can be isothermally transformed by holding the steelat a temperature within the range betWeenYIZSF-F, and 750. Peter a time period of about 5 minutes. Temperatures below the minimum of650 F. can be employed for isothermally transforming the steel to a bainitic structure;

however, when-such lower temperatures are employed the time required for transformation of the austenite to balnite becomes excessive, and in addition substantially high spheroidizing temperatures are required and for 'substantially longer periods of time, the end result being 650 Rand 900"F. for. a time period ranging between about 40minutes and about 80 minutes per inch of thickness of the steel. As was; stated hereinbefore, the steel is transformed to any one of three basic structures that the spheroidized carbide particles thus formed are extremely large, non-uniform in'size and are'erratically' While temperatures in excess of 900 F. may be employed'to transdistributed throughout the ferrite matrix.

form the steel to bainite, disadvantages are inherent in that excessively long periods of time are required to spheroidize the bainite formed. at temperatures in excess of 900 F. and the end product is non-uniform both :in carbide particle size and distribution. However, tempera-. tures much in excess of 900 F. will producea mixed.

structure of proeutectoid cementite, pearlite and bain ite' which structure is very erratic in itsresponse to the spheroidizing heat treatment and producesnon-uniform properties in the end product. A uniform dispersion of carbide particles ranging in size between pinpoint and 2.2 mm. in diameter can be produced in a steel strip having a thickness of about 0.040 inch by spheroidizing the hypereutectoid steel for about 25 minutes at;1325 F., ifthe steel istransformed-at about'650". F. However in order to produce a somewhat similar structure in this same steel which'is isothermally transformed at about 625 F. it becomes necessary to spheroidize the steel for about 60 minutes at 1325 F. Thus it is seen that by lowering the temperature 25 F. below the lower limit of 650 F. an additional 35 minutes of time is required at the spheroidizing temperature of 1325 F. This is sufficient to make such treatment economically unfeasible for continuous or semi-continuous operation. While it would appear that shorter times could be used it higher spheroidization temperatures are employed, the lower critical range of this steel is about 1360" F., thereby providing an upper limit to the spheroidizing heat treatment temperature.

In general, the steel after being isothermally transformed at a temperature between 650 F. and 900 F. is conveniently spheroidized by heating to a temperature within the range between 1300 F. and 1350 F. for

- a time period ranging between 380 minutes and 625 minutes per inch of thickness of the steel of the finish gauge which is not to be cold roll reduced during the spheroidization heat treatment. The spheroidization may be accomplished in a single-step process or a series of short time heat treatments. Thus the steel having a thickness of 0.065 inch in the isothermally transformed condition may be spheroidized for about 25 minutes at 1325 F. and thereafter cooled to room temperature. Substantially similar results can also be obtained by subjecting the steel to a multistage spheroidization heat treatment for about minutes at 1325 F., cooling the steel to room temperature, cold roll reducing to finish thickness, if such step is desired, and thereafter reheating to 1325 F. for an additional 10 minutes to complete the transformation of bainite to spheroidite. It is apparent that such multistage treatments are advantageous where the steel is to receive a series of cold roll reductions to reduce the steel to the finish thickness of the end product, because in addition to performing the spheroidizing heat treatment, the grain size is refined and the steel is also stress relief annealed between each cold rolling operation. It is preferred to spheroidize the isothermally transformed steel by heating to a temperature in the range between 1325 F. and 1350 F. for a time period of between 380 minutes and 625 minutes per inch of thickness of the steel. However, if a multistage heat treatment is to be applied to the'steel together with one or more intermediate cold roll reductions, the heat treatment time can be reduced to a time period between 160 minutes and 375 minutes per inch of thickness provided the steel has been rolled sufficiently to produce a reduction of at least in the cross-sectional area. Substantially no difference is detected when the steel is spheroidized in a single-stage operation or a multistage operation so long as the total time at temperature of the spheroidizing heat treatment is in the ranges given hereinbefore.

It is also possible to spheroidize the isothermally transformed steel at substantially shorter periods of time by making use of a somewhat different two-stage heat treatment operation. In this embodiment, the steel having a thickness not over 0.075 is heated to a temperature between 1400" F. and 1450 F. for a time period of about 5 minutes and thereafter water quenched followed by reheating to a temperature in the range between 1325 F. and 1350" F. for a time period of about 5 minutes. It will become apparent that the initial heat treatment of the spheroidizing heat treatment is slightly in excess of the lower critical temperature. This results in the formation of small amounts of austenite and the partial spheroidization of the bainite. The austenite thus formed is transformed to martensite by water quenching and the reheating at the temperature of 1325 F. to 1350 F. is effective for tempering the martensite and increasing the size of the spheroidized carbide particles formed during the initial heat treatment. While the initially formed carbide particles are submicroscopic in size when resolved at 1500 diameters it is believed that they provide nucleation points for the growth of carbide particles when the martensite which has been formed during the quench is tempered in the subsequent heat treatment. Obviously the martensite which is formed by quenching the austenite is usually found at the grain boundaries, therefore, it is not recommended that the steel be given a cold reduction while in this condition. After the second stage heat treatment and the transformation to spheroidite is complete, the steel may be cold worked in the usual way.

In order to more clearly demonstrate the process of this invention, reference may be had to the following schedule illustrating a typical processing of the steels having a carbon content of from 0.80% to 1.3% using the multistage spheroidization heat treatment. In the example, steel from Heat No. 23043E having a nominal analysis of about 1.25% carbon, 0.25% manganese, 0.15% silicon, 0.12% chromium, 0.010% phosphorus, 0.015% sulfur and the balance iron with incidental impurities was processed by standard steel mill practice to a hot rolled strip having a thickness of 0.065 inch. The steel was processed as follows:

1) Normalize hot rolled band 0.065 inch in thickness at 1500 F. to 1600 F. for 10 minutes.

(2) Austenitize at 1925 F. to 1950 F. for 5 minutes.

(3) Isothermally transform for 5 minutes at 725 F. to 750 F.

(4) Spheroidize at 1325 F. for 10 minutes.

(5) Cold roll to first intermediate gauge and anneal at 1325 F. to 1350 F. for 10 minutes.

(6) Cold roll to second intermediate gauge and anneal at 1325 F. to 1350 F. for 10 minutes.

(7) Cold roll to finish gauge and specified temper rolled hardness.

The steel had carbide particles substantially uniformly distributed throughout the ferrite matrix, the particle size ranging between pinpoint and 2.2 mm. in diameter when measured at 1500 diameters. Substantially similar results were obtained on any hypereutectoid steel of Heat 12911B containing about 1.26% carbon, 0.25% manganese, 0.10% silicon, 0.008% phosphorus, 0.011% sulfur and the balance iron with incidental impurities and Heat 23042E containing about 1.25% carbon, 0.21% manganese, 0.14% silicon, 0.010% phosphorus, 0.018% sulfur and the balance iron with incidental impurities, such heat being processed in accordance with the method of this invention.

If, on the other hand, the steel has a finish thickness, except for the amount of reduction to be applied in temper rolling after the spheroidization heat treatment, the following schedule typically illustrates the processing steps utilized on a plain carbon hypereutectoid steel having a carbon content between 0.80% and 1.3% continuously processed in a single-stage heat treatment to produce a uniform distribution of spheroidized carbide particles. In the example, steel from Heat No. 129113 having a nominal analysis of about 1.26% carbon, about 0.25% manganese, about 0.10% silicon, about 0.008% phosphorus, about 0.011% sulfur and the balance iron with incidental impurities was processed by a standard mill process to a hot rolled strip having a thickness of 0.065 inch. The steel was processed as follows:

(1) Normalize hot rolled band .065 inch in thickness at 1500 F. to 1600 F. for 10 minutes.

(2) Austenitize at 1925 F. to 1950 F. for 5 minutes.

(3) Isothermally transform for 5 minutes at 725 F. to 750 F.

(4) spheroidize at 1325 F. to 1350 F. for 25 minutes.

(5) Cold roll to finish gauge of specified temper rolled hardness.

The steel in this condition had carbide particles substantially uniformly distributed throughout the ferrite matrix, the particles ranging in size between pinpoint and 2.2 mm. in diameter when measured at 1500 diameters. Similar results were obtained by the use of this process on Heat No. 23043B and Heat No. 23042E, each of which has been identified hereinbefore.

'7 r j These steels are equally adaptable to the following process which makes use of a spheroidization heat treat.-'

m'ent at a temperature slightly in excess of the lower criticaltemperature. Heat 23042E was hot: worked to a: hot rolled band 0.040 inch thick and thereafter annealed and processed according to the following schedule:

(.1) .Normalized at a temperature in the range between 7 V 1500. F. to 1600 F, for 10 minutes.

(2') Austenitize at 1925" F. to 1950 Fnfor minutes. (3 Isothermally transform at 725 F. for 5 minutes.

(4). Spheroidize at 1400" F. to 1450 F. for 5 minutes.

( 5) Water. quench.

(6): Spheroidize at 1325 F. to 1350 F. for: 5 minutes.

This steel after the above treatment had carbide par- 7 ticles substantially uniformly distributed throughout the ferritematrix, thev particles ranging in size between pinpoint and 2.2 mm. in diameter when measured at 1500' diameter-s. Similar results were also obtained 'on the hypereutectoid steels of Heats '12911B and 23043E.

. .There are. no special skills or apparatus needed or a used to perform the process: of this-invention; All of the equipment is the normal steel mill equipment which is used in other processes. and for vdifferent results. Sub- .stantially uniform results are, obtained from heat to heat of substantially similar compositions. These results are in the range between about 650'F. and'about 900 F.

for a time period ranging between about- 40 minutes and a 80 minutes per inch of'rthickness, and spheroidizing the steel by heating to atemperature in the range between 1300 F. and 1350 FL for a time'perio d ranging between 380 minutesand 625' minutes per inch of thickness to produce a steel having carbide particles substantially uniformly distributedlin a ferrite matrix and ranging in 'size; between pinpoint and 2.2- mm. in diameter when viewed at a magnification of 1500 times.

3. In the method of spheroidizing hypereutectoid steels, the steps comprising, austenitizing a hypereutectoid steel at a temperature inthe range between about 1925 F.

and. about l950 R, isothermally transforming the aus-' tenitiz'ed steel at a temperature in. the 'range between about.725.." and about 750. F., and spheroidizing the transformed steel, at a temperature in the range :between about 1325 F. and about1350 F. to producea steel "having carbide, .particles substantially uniformly disreproducible by any one skilled in the art when using the process of-this' invention.

I We claim:.

1. In the method ofspheroidizing hyp'ereutectoid steels, the steps comprising-.austenitizing a hype'reutectoid steel at a temperature irrthe range between about 1900" F.

and about. 2000 'F.,. isothermally transforming the ausj tenitized steel at a temperature in'the range between about 650 F, and.about 900 .F., and spheroidizing the V the steps'comprising, austenitizing a hypereutectoid'steel at a temperature in therange between about 1900 F. and

about 2000 F tter a time period ranging between 60 minutes and 125:minutes per inch of thickness, isothermally transforming the austenitized steel at a temperature tributed in a, ferrite matrix and having'a carbide particle. size between'pinpoint and 2.2' mm. in diametenwhen V viewed at a magnification of 1500 times. a

4.. In the method 'ofispherodizing'hypereutectoid steel,v the steps comprising, ,aust enitizing a hypereutectoid steel at a temperature inthe range between about 1925 F. and. about 1950 F.'for. a time period ranging. between about minute's and' minutesper inch of thickness, isothermally transforming the austenitized steel at a temperature in the range between about 725 F. and about 3 750 F. for a time period ranging between about 40 minutes and about 80minutes per inch of thickness, andspheroidizing' the transformed steel at a temperaturein the rangebetween about 1325 F. and about1350 I for a time period ranging between about 380 minutes V and about 625 minutes per'inch of thickness, to produce a steelhaving carbide particles substantially uniformly. distributed in a ferrite .matrix and having a size between pinpoint. and 2.2 mm. in diameter when viewed at a magnification of 1500 times. p 7

References Cited in the file of this patent V UNITED STATES PATENTS Boyce Aug. 7, 1951 Cullen Sept..25,' s 

1. IN THE METHOD OF SPHEROIDIZING HYPEREUTECTOID STEELS, THE STEPS COMPRISING, AUSTENITIZING A HYPEREUTECTOID STEEL AT A TEMPERATURE IN THE RANGE BETWEEN ABOUT 1900*F. AND ABOUT 2000*F., ISOTHERMALLY TRANSFORMING THE AUSTENITIZED STEEL AT A TEMPERATURE IN THE RANGE BETWEEN ABOUT 650*F. AND ABOUT 900*F., AND SPHEROIDIZING THE STEEL AT A TEMPERATURE IN THE RANGE BETWEEN ABOUT 1300* F. AND 1350*F. TO PRODUCE A STEEL HAVING A CARBIDE PARTICLES SIZE RANGING BETWEEN PINPOINT AND 2.2 MM. TIMES SUBSTANTIALLY UNIFORMLY DISTRIBUTED WITHIN A FERRITE MATRIX. 